Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Tachyarrhythmia generally includes supraventricular tachyarrhythmia and ventricular tachyarrhythmia. Fibrillation is a form of tachyarrhythmia further characterized by an irregular heart rhythm. In a normal heart, the sinoatrial (SA) node, the heart's predominant natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to the atria and then to the ventricles of the heart to excite the myocardial tissues. The atria and ventricles contract in the normal atrio-ventricular sequence and synchrony to result in efficient blood-pumping functions indicated by a normal hemodynamic performance. Ventricular tachyarrhythmia occurs when the electrical impulses propagate along a pathologically formed self-sustaining conductive loop within the ventricles or when a biologic pacemaker (focus) in a ventricle usurps control of the heart rate from the SA node. When the atria and the ventricles become dissociated during ventricular tachyarrhythmia, the ventricles may contract before they are properly filled with blood, resulting in diminished blood flow throughout the body. This condition becomes life-threatening when the brain is deprived of sufficient oxygen supply. Ventricular fibrillation (VF), in particular, stops blood flow within seconds and, if not timely and effectively treated, causes immediate death. In very few instances a heart recovers from VF without treatment.
Ventricular cardioversion and defibrillation are used to terminate most ventricular tachyarrhythmias, including ventricular tachycardia (VT), and VF. An implantable cardioverter/defibrillator (ICD) is a CRM device that delivers cardioversion/defibrillation pulses, each being an electric shock, to terminate a detected tachyarrhythmia episode by depolarizing the entire myocardium simultaneously and rendering it refractory. Another type of electrical therapy for tachyarrhythmia is ATP, including atrial ATP for treating atrial tachyarrhythmia and ventricular ATP for treating ventricular tachyarrhythmia. In ATP, the heart is competitively paced in an effort to interrupt the reentrant loop causing the tachyarrhythmia. Many ICDs include both ATP and cardioversion/defibrillation capabilities.
The delivery of each cardioversion/defibrillation pulse consumes a considerable amount of power and results in patient discomfort owing to the high voltage of the shock pulses. Additionally, if delivered during the atrial vulnerable period, a cardioversion/defibrillation pulse may also cause atrial fibrillation. Thus, it is desirable to apply a cardioversion/defibrillation only when it is necessary. For example, depending on the type and origin of a tachyarrhythmia as well as the patient's own conditions, the tachyarrhythmia may be terminated with an ATP therapy and/or monitored for a certain period of time to determine whether a cardioversion/defibrillation therapy is to be delivered. However, on the other hand, when a ventricular cardioversion/defibrillation therapy is necessary, the consequence of failure to deliver the therapy timely can be fatal. Therefore, there is a need for an accurate determination of whether an anti-tachyarrhythmia therapy, such as a cardioversion/defibrillation therapy, should be delivered in response to each detected tachyarrhythmia episode.